Addison's Disease occurs when the adrenal gland cannot make normal amounts of the hormone cortisol. Addison's Disease is treated with daily doses of hydrocortisone (or related medications) to replace the body's cortisol levels. However, we do not know exactly how much hydrocortisone is the right amount for people with Addison's Disease. The purpose of this study is to measure the amount of cortisol available to the body when given different types and doses of hydrocortisone, either through the vein, or as pills.